The present invention relates generally to refractory materials, and more specifically, to insulating materials having refractory ceramic coatings. A base structure is coated with a refractory silicide coating. The coating is made of a refractory metal, i.e., those having a melting point above about 1,650xc2x0 C., and silicon. The combination of these materials forms a xe2x80x9csilicidexe2x80x9d coating.
High temperature environments such as those found in atmospheric reentry, jet turbine combustion, or rocket propulsion, necessitate the use of thermal protection systems that provide oxidation protection, high emissivity, and resistance to mechanical damage. One example of such a system is the tile used to cover the outer surfaces of the underbody and wings of NASA""s Space Shuttle Orbiter. Each tile is a lightweight, fibrous, silica-based rigid fibrous insulation unit with a nominal density of nine (9) pounds per cubic foot (pcf). The tile is made by the assignee of the present invention and designated as xe2x80x9cLI-900.xe2x80x9d
A variation of LI-900, called LI-2200, and likewise manufactured by the assignee herein, is a twenty-two (22) pcf version of the LI-900 that offers improved strength at a sacrifice in weight.
To improve strength, a new class of rigid reusable insulation was developed. These consist of the following composite ceramic materials: FRCI, AETB, and HTP. A further variation of LI-900, made by NASA""s Ames Research Center and designated xe2x80x9cFRCI-12,xe2x80x9d consists of a blend of silica and aluminoborosilicate fibers. FRCI-12 has a density of twelve (12) pcf. All three of these materials are currently qualified for use on the Shuttle Orbiter Fleet.
xe2x80x9cHTP,xe2x80x9d which stands for xe2x80x9chigh thermal performance,xe2x80x9d refers to a new class of lightweight ceramic material introduced by the assignee herein around 1982. Basically, this high-strength insulation is produced by fusing silica and alumina fibers together. The insulation is produced to a number of standard densities at a standard composition of twenty-two (22) percent. The HTP family of insulants has yielded improvements in strength and maximum temperature capability relative to earlier generations of ceramic insulation. Also, at about the time HTP was introduced, NASA (Ames Research Center) introduced the rigid fibrous insulation material known as xe2x80x9cAETB.xe2x80x9d
Coatings have been used in conjunction with refractory metals, such as tantalum, niobium, and molybdenum, to protect the underlying metallic structures from oxidation and erosion experienced in high temperature propulsion environments. Silicide coatings have been used in the past for such purposes.
The TPS tiles noted above have in the past been protected by application of reaction cured glass (RCG) coatings. These coatings were developed in the early 1970""s for the LI-900 class of thermal insulants. RCG is composed mostly of silica with a small amount of silicon hexa/tetraboride added as a blackening agent and a fluxing agent. The coating is applied as a 8-12 mils thick layer onto the surface of a ceramic tile. As a surface coating, RCG has relatively poor resistance to impact; as a silica-based system, RCG""s maximum temperature capability is limited to its softening point of about 2,700-2,800xc2x0 F.
In 1989, NASA""s Ames Research Center developed an insulation product called toughened uni-piece fibrous insulation (TUFI). The coating was still silica-based, but it contained about twenty (20) percent molybdenum disilicide as a blackening agent. TUFI products represented an advancement in the state of the art because the coating is applied as a surface impregnation, meaning that it became commingled with the fibers of the insulation tile near the surface region. The resultant fused coating is a fiber reinforced glass which is much more durable than the RCG coating. As a silica-based coating, however, the TUFI product has the same upper temperature limit as RCG, i.e., 2,700-2,800xc2x0 F. TUFI has been successfully applied to FRCI, HTP, and AETB.
Refractory metal coatings have been used in ceramic applications. For example, U.S. Pat. No. 5,413,851 to Storer describes a ceramic carbon fiber coated with a refractory metal or metal-based ceramic material. The refractory metal materials used include molybdenum, tantalum, tungsten, niobium, oxides of aluminum, yttrium, zirconium, hafnium, gadolinium, titanium, erbium and other rare earth metals. The fibrous materials that are coated include alumina, alumina-silica, and alumina-boria- silica. The coatings are used to enhance strength.
U.S. Pat. No. 4,530,884 to Erickson et al. describes a ceramic-metal laminate which is used as insulation in high temperature environments. The composites described therein have an inner ceramic layer and an outer metal layer and an intermediate interface layer of a low modulus metallic low density structure. These composites are principally used as turbine blades of gas turbine engines.
U.S. Pat. No. 5,863,846 to Rorabaugh et al. describes a ceramic insulation used in aerospace applications in which a slurry is molded from ceramic fiber to form a soft felt mat which is impregnated with a sol prior to drying. The mat is exposed to a catalyst that diffuses into the mat and causes the sol to gel.
U.S. Pat. No. 5,814,397 to Cagliostro et al. describes ceramic materials used in space re-entry vehicles in which silica coatings are formed on fibrous insulations. U.S. Pat. No. 5,079,082 to Leiser et al. describes a porous body of fibrous, low density silica-based insulation material that is impregnated with a reactive boron oxide-containing borosilicate glass frit, a silicon tetraboride fluxing agent, and a molybdenum silicide emittance agent.
A continuing need exists for improved lightweight thermal insulation materials that are temperature resistant and physically durable. In particular, improved coatings, such as those described below, are needed for all of the advanced rigid, fibrous insulation materials described above, such as FRCI, HTP, and AETB.
An object of the present invention is to provide an insulative material that exhibits oxidation protection, high emissivity, and resistance to mechanical damage.
Another object of the present invention is to provide an insulative material that is capable of withstanding high-temperature environment, including those associated with atmospheric reentry, jet turbine combustion, and rocket propulsion.
Still another object of the invention is to provide a silicide coating that exhibits higher temperature capability than silica-based coatings.
These and other objects of the invention are met by providing a thermal protection system comprising a base structure having at least one surface, the base structure being made of a ceramic oxide material, and a coating formed on the at least one surface, the coating comprising a refractory metal and silicon and being at least partially infiltrated into the base structure at the at least one surface.